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Mirror image gives 'spectacular' view of JET's plasma | 02/08/2018

Remarkable views of the JET plasma are now being achieved – from cameras located outside the machine hall!

New cameras installed on the exterior of JET's bioshield wall use mirrors to capture detailed views from the heart of the plasma (see video).

A sophisticated engineering project has moved the diagnostics camera system out of the Torus Hall to prevent neutron damage in high power Deuterium-Tritium operations. The system will be commissioned over the coming months.

Multiple camera systems covering two separate lines of sight – which help scientists discover more about plasma behaviour and its effect on components of the tokamak – now lie behind the bioshield following the CDT-2 project (Cameras Compatible with Deuterium-Tritium Operation, Phase 2). Two optical relay systems ensure the images are reflected to the camera systems (40m and 30m away of optical path respectively from their previous location) and that the picture isn't hindered by the 3 metre-thick bioshield.

Without the cameras being relocated, any live pictures from inside the vessel would be "as clear as a white cat in a snow storm", according to Guy Matthews, lead scientist on the project.

Past experience suggests that during JET's forthcoming Deuterium-Tritium operations, the neutron dose will be such that the cameras would only last for one high performance pulse if left in their previous locations.

The mostly EUROfusion-funded project saw engineers drill two openings in the bioshield wall to allow clear lines of sight. The perspectives from the two groups of cameras are known as Wide Angle View (WAV), which views the cross section of the tokamak at Octant 5, and Divertor View (DIR) which covers the divertor tiles at the same octant.

Camera diagnostics in JET are used for variety of functions – whether that be operational, for machine protection, or for scientific reasons. For operation, the experiment session leaders need views of the plasma during a pulse and measurements of the temperature of the in-vessel plasma-facing components to ensure there is no damage to the materials. Scientific cameras are used for the simulation of plasmas, in addition to monitoring impurities and measuring heat fluxes to in-vessel components.

A new three storey optical lab has been attached to the Torus Hall wall to house the new camera clusters, while, in the hall itself, new structures have been built to support the large relay mirrors which enable the light beams to weave their way around existing JET equipment.

Three EUROfusion associations have been involved in the project – CCFE, CIEMAT (Spain) and the Wigner Research Centre for Physics (Hungary).

Engineering analysis for the supporting structures and mirror holders was carried out by John Williams with support from Ruben Otín and Neelam Gupta.

Itziar Balboa, who leads the viewing and thermal measurements group and was tasked with leading the CDT-2 team from July 2015, said: “It is like looking down a rabbit hole from across a field but with lots of trees in the way.

“A thorough stress analysis was carried out to assess the suitability of the supporting structures and mirror holders including the level and impact of eddy forces.”

Eddy currents are induced by the magnetic fields and can distort the mounting frames and cause unacceptable movement of the mirror relays. It was essential to produce a design which was compatible for all sorts of operational scenarios within JET.

The first data has already been recorded in the first JET pulses of 2018. Scott Silburn, who is responsible for the fast cameras and wide angle infra-red, said: “The first images from CDT-2 are quite spectacular and there's a lot excitement about what we do with the new capabilities in the upcoming JET experiments.”